1. Field of the Invention
This invention generally relates to a linear motion guide unit, and, in particular, to a rolling contact type linear motion guide unit capable of providing an improved straightness in linear motion.
2. Description of the Prior Art
A rolling contact type linear motion guide unit is well known in the art and it generally includes a slider and a rail 8 as shown in FIGS. 3 and 4. In the conventional guide unit shown in FIGS. 3 and 4, the slider 1 includes a race 2, which is generally in the shape of a horse saddle, and a circulator 3 which is located inside the space defined by the race 2. That is, the race 2 is made out of a thin sheet metal by bending it in a desired form as shown so that it has a generally U-shaped cross section excepting its front and rear end sections. Since the race 2 has a generally U-shaped cross section, it has a flat horizontal section and a pair of vertical sections, each depending from a corresponding side of the horizontal section, and an outer guide groove 5 is provided at an outer side surface of each of the vertical side sections.
The circulator 3 in the illustrated example is comprised of a pair of upper and lower half sections, which are both fixedly attached to the race 2 to construct the slider 1. The circulator 3 is provided with a pair of endless circulating paths, each including a load path section, a return path section 7 and a pair of curved connecting path sections 6 connecting the corresponding ends of the load and return path sections. The load path section is partly defined by the inner guide groove 5 as will become clear later. A plurality of rolling members or balls 4 in the illustrated example are provided in each of the pair of endless circulating paths.
On the other hand, the rail 8 is elongated in shape and has a generally U-shaped cross section so that it includes a bottom section and a pair of vertical sections, each extending upwardly from a corresponding side of bottom section. Typically, the rail 8 is made out of a thin sheet metal by bending it in a desired shape as shown. The rail 8 is provided with an outer guide groove 9 at an inner side surface of each of the vertical sections and a corresponding inner guide groove 5 is brought into alignment with an associated one of the outer guide grooves 9 when the slider 1 is assembled with the rail 8. Thus, when assembled, the load path section of each of the endless circulating paths is defined by a pair of associated inner and outer guide grooves 5 and 9. As a result, the balls 4 located in the load path section are partly received in both of the inner and outer guide grooves 5 and 9 so that a rolling contact is provided between the slider 1 and the rail 8. In addition, since the balls 4 may roll along each of the pair of endless circulating paths endlessly, a theoretically infinite relative linear motion may be provided between the slider 1 and the rail 8.
However, when manufacturing a linear motion guide unit as, for example, shown in FIGS. 3 and 4, the rail 8 is fabricated by bending a relatively thin sheet metal into a desired shape as shown in FIGS. 3 and 4. Then, an intermediate rail thus formed by bending is subjected to heat treatment. In this case, however, it is often the case that a thermal stress is created in the intermediate rail unevenly so that the resulting rail 8 tends to become warped as indicated by the solid lines from an intended straight state indicated by the two-dotted lines in FIG. 3. Such a distortion, of course, deteriorates the degree of straightness of the rail 8 and it could impair the overall performance of the linear motion guide unit. It is true that corrective tempering could be applied to mitigate the distortion, but it only allows to lessen the distortion and does not allow to remove the distortion. Besides, when the rail 8 is required to be relatively long, such an approach is less than satisfactory.